U.S. Senate seats in Connecticut open rarely. One of those opportunities for change occurs this year, with the retirement of independent DemocratJoseph I. Liebermanafter four terms — 24 years — in office.

The Courant's editorial board endorses U.S. Rep. Chris Murphy, 39, of Cheshire as the Democrat who is best suited to replace Mr. Lieberman and urges his nomination in the party primary on Aug. 14. (Linda McMahon and Chris Shays face each other in a primary election for the Republican Senate nomination on the same date.)

This endorsement is for the primary only; the editorial board will reassess the Senate race before the Nov. 6 general election.

Mr. Murphy's primary opponent is former Secretary of the State Susan Bysiewicz, 51, of Middletown. These two veteran officeholders agree on many issues: for example, support for President Obama's Affordable Care Act, ending the war in Afghanistan now, stronger gun controls, support for Wall Street reforms and consumer protection, greater investment in green technology and alternative energy, and help for stressed mortgage holders.

Both of them are capable and knowledgeable, both driven and ambitious, both willing to take risks.

Why Chris Murphy?

Experience and temperament are Mr. Murphy's trump cards. His three terms in the U.S. House representing the central-northwestern Connecticut 5th District give him a leg up on mastering the Senate learning curve.

Temperamentally, the mostly agreeable Mr. Murphy, if he is elected, would be more likely than the sharp-edged Ms. Bysiewicz to work effectively within the Democratic caucus as well as across the aisle to deliver for Connecticut and the country.

Has Mr. Murphy been effective as a member of the House? Yes, at least modestly so, which is all that can be said about any member at a time when Congress is measured more by what it doesn't do than what it does.

One of the big things Congress accomplished during Mr. Murphy's time in the House was to extend health coverage to millions of Americans who didn't have it. That was a signal accomplishment, and Mr. Murphy was right at the heart of the action, helping to push the Affordable Care Act across the finish line.

He is one of Congress' leading authorities on health care — as he was as co-chairman of the legislature's public health committee and author of Connecticut's stem cell research act when he was a state senator.

Among bills Mr. Murphy introduced as a member of Congress that were signed into law were measures that will triple the number of supportive housing units for the non-elderly disabled nationwide and that appropriated $15 million to acquire and clean up the Waterbury Industrial Commons. Other Murphy legislation puts Congress officially on record in support of the New Haven-Springfield commuter rail line and sends federal grants to help the state purchase and preserve threatened landscapes in Litchfield County.

Why Not Ms. Bysiewicz?

Ms. Bysiewicz is a formidable politician, a punishing campaigner who has won four statewide elections. As a state representative, she helped to secure hospital coverage for women who have mastectomies. As secretary of the state, she modernized the office.

But she is erratic. Her unimpressive handling, as the state's top election official, of the 2010 election — including a too-early call ofDannel P. Malloyas the gubernatorial winner — provoked national ridicule. Her use of the state computer system to store and use private political data, while not illegal, was highly improper.

All in all, Mr. Murphy is the better of the two Democratic candidates.

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